It is well known in the optical arts that optical surfaces are most perfectly formed when the optical material is glass. The single crystalline structure of glass permits grinding, lapping and polishing of the surface to a degree of perfection which can be measured in terms of single wave lengths of light. In this manner highly accurate lenses are produced.
In the present state of the art these glass lenses are used as master dies in various replication processes. One such process is the production of thermosetting polycarbonate lenses which are widely used in presecription eyeglasses. These glass dies must be used because machined metal or plastic dies cannot approach the optical quality of the glass surface. The polycarbonate curing process requires 24 hours with the result that many identical glass dies are required for volume production.
Unfortunately, the plastic replication process is very abusive of the delicate optical glass. The thermal shock due to the heat involved in the thermosetting process often cracks the master. A common occurrence is that the plastic will not separate from the glass die, and wedges and hammers must be used to effect separation. Such treatment results in chips and scratches in the glass master, rendering it useless as a die. These failures of the glass masters represents a significant cause of production problems in plastic lens manufacturing, and an important cost factor.